Cytochrome b and

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

Complex III 280 kDa 11 28 type hemes (b and bg) bound to same mitochondrially coded peptide 1 C heme (cytochrome c,) 1 Fe-S center Rieske factor Spans membrane, cytochrome b, and b in membrane, cytochrome c, and Fe-S center on outer face 0.25-0.53 Pumps protons out of matrix during electron transport/2e"... 

Barbiturates such as amobarbital inhibit NAD-hnked dehydrogenases by blocking the transfer from FeS to Q. At sufficient dosage, they are fatal in vivo. Antin cin A and dimercaprol inhibit the respiratory chain between cytochrome b and cytochrome c. The classic poisons H2S, carbon monoxide, and cyanide inhibit cytochrome oxidase and can therefore totally arrest respiration. Malonate is a competitive inhibitor of succinate dehydrogenase. 

Edwards, S.W. and Lloyd, D. (1988). The relationship between superoxide generation, cytochrome b and oxygen in activated neutrophils. FEES Lett. 227, 39-42. 

Figure 13.12 The protonmotive Q cycle. Electron transfer reactions are numbered and circled. Dashed arrows designate movement of ubiquinol or ubiquinone between centres N and P and of the ISP between cytochrome b and cytochrome c,. Solid black bars indicate sites of inhibition by antimycin, UHDTB and stigmatellin. (From Hunte et al., 2003. Copyright 2003, with permission from Elsevier.)...

The cytochrome b and flavin content and properties of the 02 -forming NADPH oxidase solubilized from activated neutrophils. Biochim. Biophys. Acta 746, 40-7. 

A study of 25 patients with chronic granulomatous disease A new classification by correlating respiratory burst, cytochrome b, and flavoprotein. J. Clin. Immunol. 6, 136-45. 

The influence of phenobarbital on the turnover of hepatic microsomal cytochrome b, and cytochrome P-1+50 in the rat. Biochim. Biophys. Acta (1970) 201, 20-05-... 

The reduction is mediated by a PCE reductase and hydrogenase, both of which are associated with the cytoplasmic membrane. The hydrogenase faces the periplasm of the cell and thus releases the protons into the periplasm (Schumacher and Holliger 1996). Two electrons are transferred across the membrane to the PCE reductase, via cytochrome b and menaquinone (Fig. 9.13). Since dihydrogen is the only electron donor used by Dehalobacter restrictus, the hydrogenase is a crucial enzyme of this... 

Fe Cytochrome oxidase reduction of oxygen to water Cytochrome P-450 0-insertion from O2, and detoxification Cytochromes b and c electron transport in respiration and photosynthesis Cytochrome f photosynthetic electron transport Ferredoxin electron transport in photosynthesis and nitrogen fixation Iron-sulfur proteins electron transport in respiration and photosynthesis Nitrate and nitrite reductases reduction to ammonium... 

Cytochrome c There are many cytochhomms, which differ in slight detail, but these are broadly grouped together as cytochrome a, cytochrome b and cytochrome c. The active centre of the cytochrome is the heme group. It consists of a porphyrin ring chelated to an iron atom. The porphyrin ring... 

Hamers, M. H., et al. Cytochrome b and the superoxide-generating system in human neutrophils, in Biological and Clinical Aspects of Superoxide and Superoxide Dismutase, (eds.) Bannister, W. H., Bannister, J. V., pp. 242, New York, Elsevier 1980... 

Complex III crystallizes in two distinct conformations (not shown). In one, the Rieske Fe-S center is close to its electron acceptor, the heme of cytochrome c, but relatively distant from cytochrome b and the QH2-binding site at which the Rieske Fe-S center receives electrons. In the other, the Fe-S center has moved away from cytochrome c, and toward cytochrome b. The Rieske protein is thought to oscillate between these two conformations as it is reduced, then oxidized. 

B) Coproporphyrin III. Note that a different tautomeric form is pictured in B than in A. Tautomerism of this kind occurs within all of the porphyrins. (C) Protoheme, the Fe2+ complex of protoprophyrin IX, present in hemoglobin, cytochromes b, and other proteins. 

In bacteria some cytochromes b and dt serve as terminal electron carriers able to react with 02, nitrite, or nitrate, while others act as carriers between redox systems.141-1433 The aldehyde heme a is utilized by animals and by some bacteria in cytochrome c oxidase, a complex enzyme whose three-dimensional structure is known (see Fig. 18-10) and which is discussed further in Chapter 18. 

Three are the large functional subunits of cytochrome oxidase, one is cytochrome b, and seven are subunits of the NADH dehydrogenase system (Complex I). 

Chloroplasts come in various sizes and shapes, but all contain a small number of DNA molecules ranging in size from 120-160 kb. Complete sequences are known for DNA from chloroplasts of a liverwort (121,025 bp),246 tobacco (155,844 bp),247 maize (Zea mays),248 and other plants. The 140,387 bp DNA from maize chloroplasts is a circular molecule containing the genes for 23S, 16S, 5S, and 4.5S RNA, for 30 species of tRNA, and for 70 different proteins. Among them are subunits of RNA polymerase, NADH dehydrogenase, subunits of both PSI and PSII, rubisco (large subunit), cytochromes b and/, six subunits of ATP... 

Complex III The Cytochrome hcl Complex. The cytochrome bc1 complex contains two b-type cytochromes b, and bH. Both of the b hemes are on a single 30-kd polypeptide. The complex also contains cytochrome ct, an iron-sulfur protein, and between four and six additional subunits. 

An unusual [2Fe-2S] ferredoxin with unique spectroscopic properties exists in association with cytochromes b and c, and is involved in respiratory electron transport in mitochondria, chloroplasts and certain bacteria. When isolated, the complex contains two b hemes, one c, heme and the 2Fe-2S protein. The 2Fe-2S protein from the bct complex (Sections 62.1.5.2.3 and 62.1.5.2.5) was purified from bovine mitochondria by Rieske et al.,162 and is referred to as the Rieske iron-sulfur protein. The properties of this protein have been reviewed763 and its topography in mitochondrial ubiquinol-cytochrome c reductase has been described.764 They have high redox potentials in the range+150-330 mV. 

This reaction is reminiscent to the autoxidation of cytochrome b5 and described reductions of cytochromes b and c with hyperoxide. In order to study further analogy of osmochrome systems to the cytochromes, water-soluble osmochromes have been prepared from sulfonated precursors. Due to the acidity of pure water, osmochrome systems cannot be held in water, but the osmichrome salt Na3[Os[TPPS4) (1-Meim)2] was isolated [51]. 

The sites of action of some of the commonly used inhibitors of the electron-transport chain are shown in Fig. 14-3. These sites have been established by application of the crossover theorem (Chap. 10). For example, the fungus-derived antibiotic antimycin A causes an increase in the level of reduced cytochrome b and a decrease in the level of reduced cytochrome C] (i.e., an increase in the level of oxidized cytochrome c() thus, it is inferred that antimycin A interacts with complex III. 

The absence of ADP is acting, in effect, as an inhibitor of electron transport, for reasons discussed in Prob. 14.6 below. Hence, by application of the crossover theorem (Chap. 10), there are large differences in the reduction of sites of the electron-transport-chain between NAD and coenzyme Q, between cytochrome b and cytochrome c, and between cytochrome c and cytochrome a. Therefore, the absence of ADP must be inhibiting electron transport at these points in fact, these are the sites of proton extrusion leading to ATP synthesis during electron transport. 

A cycle for substrate reduction can be devised by beginning at the reduced Mo(IV) state and reversing the direction of OAT and CEPT steps, as illustrated for the nitrate reductase reaction in Fig. 9(b). Electrons are provided to reduce Mo(VI) to Mo(IV) via cytochrome b and FAD. 

---